Several kinds of networks are known in which plural nodes or stations gain access to a common transmission medium. Examples of such systems are single bus networks with collision detection, and token ring networks as well as token passing bus networks. Recently, networks providing another technique of multiple access to a common transmission medium have become of interest. These are systems comprising two parallel busses with counterflowing transmission of information on the two busses. Slots are released at regular intervals by headend stations, and these slots are used by the node stations for data transmission. Each node station has to request access to a slot by previously transmitting an access request in an Access Control Field of a passing slot. It keeps a count of access requests it has seen from other stations (located upstream in request transmission direction) before it raises its own access request, and lets as many free slots pass by (for use by the other stations) as the count indicated, before it occupies the next free slot for transmission of its own data.
Such systems were described e.g. in a paper by R. M. Newman et al. entitled "The QPSX MAN", published in the IEEE Communications Magazine, Vol. 26 No. 4 (April 1988) pp. 20-28; and in a Draft Proposed IEEE Standard 802.6 entitled "Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN)", Draft D6, Nov. 15, 1988.
Though these known distributed queueing systems are well suited for networks comprising a limited number of stations, they have some disadvantages which become unacceptable and may render the system inefficient when the number of stations is raised to several hundred, and when the length of the transmission busses is in the order of several kilometers.
These disadvantages are in particular: An "unfairness" for some stations with respect to others, due to the fact that each station has to await a free access request field before it can transmit a request, so that stations located upstream (in request transmission direction) are preferred; and an impossibility to guarantee the availability of a sequence of consecutive slots for one station desiring to transmit the portions of a data packet without interruption.